In current maize, or corn, production in the United States, the vast majority of the seed maize sold to and planted by commercial farmers are single-cross, Fl hybrid varieties. The current commercial techniques for producing hybrid maize varieties require that predictable cross-breedings be achieved between designated male and female parent plants of specific inbred genealogies. Thus the common practice is to plant seeds of the designated male and female lines together in a common field so that pollen from the male parent plants can travel and pollinate the female parent plants.
This procedure is facilitated by the hermaphroditic character of male plants. Each plant has separate male and female inflorescences. Thus either line may be selected as the male or female parent. However because the plants are normally hermaphroditic, in order to ensure that a proper cross is made between the desired male parent and the desired female parent plant, it is necessary to ensure that pollen from the female parent plant does not self-pollinate that same plant or pollinate a sibling female parent plant. In order to ensure that such undesirable self-pollination or sibling-pollination does not occur, the common practice in the hybrid seed maize industry is to physically remove the male inflorescence from the designated female parent plants by detasselling the designated female parent plants by hand. While mechanical devices are presently available to detassel the female parent plants, because of the variability in size of maize plants in any given field and the necessity for not cutting too much of the maize plant away, mechanical processing is not efficient, and thus the detasselling procedure is conventionally done by hand, sometimes in combination with a mechanical device. This process is a very labor-intensive activity and is very concentrated in its time period, normally a time period of four to six weeks in June, July, or August, in the Northern Hemisphere, because the activity must be performed at closely spaced intervals during the flowering period of the maize inbred used as a female parent. This detasselling operation is both a difficult logistical operation, because of the need to acquire large amounts of short-term labor, and is an expensive process because of its labor intensiveness.
Accordingly, much effort has been spent over time to develop maize plants which are male-sterile. The term male-sterile generally designates a plant wherein the male inflorescence on the female parent produces no viable pollen, but the plant still has complete female reproductive capability. The use of a male-sterile maize plant in the hybrid production system avoids the need for detasselling, since the only pollen available for the airborn pollination of the designated female plants, which are male-sterile, is the pollen produced by the designated male parent plant. In this way predictable crosses can be made so that hybrid progeny suitable for field use can be created. Unfortunately, the use of male-sterile maize plants has previously had several inherent disadvantages.
There are two categories of presently known and commercially utilizable systems for maintaining male-sterile stocks of maize plants. One system relies on a so-called cytoplasmic, or non-nuclear, male-sterile trait, and the other system relies on genic, or nuclear chromosomal, trait inheritance to maintain male sterility.
The cytoplasmic male sterility system relies on genes not contained in the nucleus of cells, hence the name. This system is more properly termed cytoplasmic-nuclear, since it depends on both the presence of a cytoplasmic male-sterility gene and the absence of a nuclear restorer gene which can condition restoration of fertility. Since cytoplasmic genetic material is normally transmitted solely from the female parent plant in maize, and is only very rarely, if ever, passed through pollen, the use of a cytoplasmic male-sterile trait in a female parent plant allows pollen to be donated by a male-fertile parent while the resulting progeny plants are reliably male-sterile because of the cytoplasmic gene contribution of the female parent plant. One system disclosed for use of cytoplasmic male sterility to produce commercial hybrid maize seed is disclosed in U.S. Pat. No. 2,753,663.
For a time the United States hybrid seed industry utilized cytoplasmic male-sterile maize lines for the production of hybrid maize seed. The most popular type of cytoplasmic male sterility was referred to as the Texas-Sterile or T-Sterile cytoplasm. This cytoplasmic sterility was used widely in producing several types and varieties of hybrid seed maize for sale until 1970, when there occurred an epiphytotic of a race of T-type Helminthosporium maydis causing a form of southern leaf blight in most of the then-existing male-sterile plants and hybrids produced from them. This event convinced many maize breeders that cytoplasmic male-sterility was an inherently inappropriate mechanism for achieving male-sterile plants, since the differences between normal cytoplasm and that carrying male-sterility also seem inherently to affect not only pollen fertility but also disease susceptibility. In addition, the heavy damage caused by this epiphytotic event has created a widespread consumer reluctance to use cytoplasmic male-sterile lines because of fears about reoccurrence of epiphytotic events in the other cytoplasmic male-sterile lines. To date, two other such cytoplasmic male-sterile lines have been identified. Referred to as the C and S types, these types have inherent problems of stability and sterilization of inbred lines, in addition to the consumer and breeder reluctance to use a cytoplasmic male-sterile system. This reluctance, and concerns about epiphytotic events, may be an inevitable consequence of cytoplasmic male-sterility, because the cytoplasmic traits are inherently passed from the female parent in hybrid seed production and therefore the hybrid maize seed produced from a production system using cytoplasmic male-sterile genes must, of necessity, carry the cytoplasmic traits of the male-sterile female parent. In other words, there is no mechanism available using such a system to dominate or mask any undesirable traits carried in this male-sterile cytoplasm, thus ensuring that whatever deficiencies, disease susceptibility, or other traits that are carried in the male-sterile cytoplasm will also be carried in the hybrid seed sold to farmers and the plants resulting from the seed.
The other approach to male-sterility in maize plants is genic male sterility in which the chromosomal nuclear genes of the maize plant cause the male-sterility. Much work has been done on identification of the male-sterile qenes in maize, and, to date, at least nineteen different nuclear gene mutations are known which can produce male-sterility. See the list of male-sterile genes, for example, in Column 15 of U.S. Pat. No. 3,861,079. In every presently known inheritable trait which produces male sterility, the sterility is determined by a single gene, and the allele for male-sterility is recessive. The known male-sterile genes have been mapped extensively and the chromosome number and map position of all presently identified genes are well characterized. The possibility of using genic male-sterile lines has long been available to producers of hybrid seed but has not proved sufficiently practical for common use.
The difficulty in the use of conventional genic male-sterile lines arises from the fact that it is difficult to maintain an inbred stock which is homozygous for the recessive allele giving rise to male-sterility. The reason for this is simply that plants carrying the homozygous trait for male-sterility are incapable of producing the pollen necessary to self-pollinate or pollinate siblings also homozygous for the recessive allele. It is, of course, possible to cross-pollinate male-sterile plants homozygous for the male-sterile recessive allele with pollen from male-fertile plants which are heterozygous for the male-sterile gene (i.e. having in their allelic pair one male-fertile allele and one male-sterile allele Ms/ms). The progeny from such a cross-breeding are approximately fifty percent male-sterile and approximately fifty percent male-fertile. Thus, additional homozygous male-sterile plants can be created, but only in a field fifty percent populated by heterozygous male-fertile plants. This system of male-sterility is thus impractical for use in hybrid maize production, since the best that could be expected through the use of such plants is that fifty percent of the designated female parent plants intended for use in the hybrid seed production stage would be male-sterile. Therefore a detasselling operation would be necessary, in any event, to detassel the remaining fifty percent of the plants. Since detasselling is thus necessary in any event in a field utilizing this procedure, there is little commercial advantage in using this process, and it is not widely used at present.
It has been previously noted that certain genes are linked very closely to male-sterile genes. For example, the male-sterile-1(ms.sub.1) gene is closely linked for the yellow/white endosperm gene locus. By maintaining stocks of homozygous pollen sterile and white endosperm plants, it is possible to cross these stocks with heterozygous stocks and selecting for male-sterile plants by endosperm color. This procedure is effective, except that the recombinants that do occur will give rise to fertile plants which must be rogued out in the field. The method has not gained widespread use because of the plants produced from these recombinations.
Other more sophisticated systems have been developed to attempt to create genic male-sterile maize plants for use in hybrid seed production. An example of such technique is disclosed in U.S. Pat. No. 3,710,511 and U.S. Pat. No. 3,861,709 to Patterson. That technique utilizes reciprocal translocations and various forms of chromosome deficiencies and duplications to produce male-sterile stocks. This procedure illustrates the complexities envisioned as normally required in creating genic male-sterile maize lines.
One variation of the system of the present invention makes use of one example of a class of genic elements known as transposable elements. Transposable elements, also known as transposons, are genic elements which can spontaneously relocate themselves from one locus to another in a chromosome or to any other chromosome located in the plant genome. Transposable elements were first identified in maize, in the pioneering work of Dr. Barbara McClintock. Several systems of transposable elements have been identified by Dr. McClintock initially, and by others subsequently. Among the systems of transposable elements identified by McClintock is the suppressor-mutator (Spm) system. Spm has a transposition-competent (autonomous) element which encodes the information enabling the excision of the Spm element from one location in the genome and reintegration in another location. The Spm system can affect the expression of the locus into which it is inserted. The Spm system sometimes exhibits an expression phenomenon Which has been modeled as a two element system. In modeling the Spm system as a two element system, one element has been referred to as a receptor, Rs and the other element referred to as the suppressor Sp. It has been discovered that the suppressor can be separately located from the receptor and still cause the receptor to repress expression of a gene in which it is inserted. The second component has now been shown to be a defective Spm element in which a portion of the DNA sequence of the Spm has been deleted (Pereira et al., 1985) and which has concomitantly lost the ability to catalyze its own transposition, but which can still be induced to transpose when an intact Spm is present elsewhere in the genome. The second component can be termed a defective Spm (dSpm), or alternatively a receptor factor (Rs), or (I), both definitions describing an inserted and stable genetic factor which suppresses expression of the gene into which it is inserted when the Spm suppressor function is also located in the plant genome.